<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>:focus伪类选择器</title>
    <style>
        input:focus {
            background-color: pink;
            border: 5px;
            color: aqua;
        }
    </style>
</head>

<body>
    <input type="text" value="第一个框">
    <input type="text" value="第二个框">
    <input type="text" value="第三个框">
</body>

</html>